(Front tire Dia. (roll out) X rear gear ratio) Divided by the rear tire Dia. (roll out) = front gear needed.
33" tire x 6.00 gear = 198
198 divided by 40" tire = 4.95 gear (4.88 gears would give him less then 1% overdrive) most like around 6-10% overdrive (a 4.56 gear would give around 8%)
To be more accurate, it is best to use formula using roll out of the tires on the vehicle rather then just tire diameter.
i always run as close to straight up as possible maybe thats why mine drives like chit